The patterning of radiation sensitive polymeric films with high energy radiation such as photons, electrons, or ion beams is the principle means of defining high resolution circuitry found in semiconductor devices. The radiation sensitive films, often referred to as photoresists regardless of the radiation source, generally consist of multicomponent formulations that are coated onto a desired substrate such as a silicon wafer. The radiation is most commonly ultraviolet light at wavelengths of 436, 365, 257, 248, 193 or 157 nanometers (nm), or a beam of electrons or ions, or ‘soft’ x-ray radiation, also referred to as extreme ultraviolet (EUV) or x-rays. The radiation is exposed pattern wise and induces a chemical transformation to occur that renders the solubility of the exposed regions of the films different from that of the unexposed areas when the films are treated with an appropriate developer, usually a dilute, basic aqueous solution, such as aqueous tetramethylammonium hydroxide (TMAH).
Typical photoresists contain a polymeric component and are generally comprised of a polymeric matrix, a radiation sensitive component, a casting solvent, and other performance enhancing additives. The highest performing photoresists in terms of sensitivity to radiation and resolution capability are “chemically amplified” photoresists, allowing high resolution, high contrast and high sensitivity that are not generally provided by other photoresists. Chemically amplified photoresists are based on a catalytic mechanism that allows a relatively large number of chemical events such as, for example, deprotection reactions in the case of positive photoresists or crosslinking reactions in the case of negative tone photoresists, to be brought about by the application of a relatively low dose of radiation that induces formation of the catalyst, often a strong acid.
The absorbance characteristics of the polymeric matrix also impacts the suitability of a given photoresist for exposure with particular radiation sources. The choice of a polymer must be carefully considered when designing a material for lithographic applications, particularly where such polymers are to provide a relatively transparent matrix for radiation-sensitive compounds such as photoacid generators (PAGs). Absorbance characteristics are important because the wavelength of radiation used in optical lithography is directly proportional to the ultimate resolution attainable with a photoresist. The desire for higher resolution has therefore led to to the use of shorter and shorter radiation wavelengths. For example, the phenolic polymers used for 248 nm imaging, namely derivatives of poly(4-hydroxystyrene)(PHS), are unsuitable for use with 193 nm radiation as the opacity of these PHS materials at 193 nm does not allow for sufficient radiation to create an appropriate image profile throughout the photoresist film thickness. As a result, the selection of particular materials is necessary for each wavelength of optical radiation used.
The dissolution characteristics of photoresist materials in a developer are also important considerations. The semiconductor industry has largely supported the use of TMAH as a developer for photoresists. PHS materials, e.g., tend to dissolve very uniformly in TMAH without swelling. Additionally, the rate at which the polymeric films dissolve can be tuned, for example, by the use of protecting groups and dissolution inhibitors in positive tone photoresists, and by effective crosslinking and other functionalization in negative tone photoresists. Uniform dissolution has been a difficult property to incorporate into new photoresist materials, however, especially those designed specifically for 193 nm imaging. The current polymer resists for 193 nm imaging, such as acrylic acid derivatives, cyclic olefins and alternating cyclic olefin-maleic anhydride-based materials, generally fall into this category of nonlinear dissolution. In fact, these materials often exhibit significant swelling during the initial stages of development, making the development of photoresists based on these materials difficult.
Alternative materials based on fluoroalcohols have been previously proposed as a means of providing aqueous base solubility. See, e.g., Ito et al. (2001), “Polymer Design for 157 nm Chemcially Amplified Resists,” Proc. SPIE 4345:273–284; Kunz et al. (2001), “Experimental VUV Absorbance Study of Fluorine-Functionalized Polystyrenes,” Proc. SPIE 4345:285–295; and Bae et. al. (2001), “Rejuvination of 248 mn Resist Backbones for 157 nm Lithography,” J. Photopolym. Sci. Tech. 14:613–620. Examples of such materials include norbornene hexafluoroalcohol, styrene hexafluoroalcohol and cyclohexyldodecylfluoro-alcohol-based polymers. While each of these platforms provides base-soluble materials, each has disadvantages for commercial high resolution photoresist applications. For example, the norbornene hexafluoroalcohol monomer requires special polymerization conditions, such as ring-opening polymerization, transition metal catalyzed addition polymerization, or alternating free-radical polymerization with a comonomer such as maleic anhydride. Thus, this monomer does not accommodate a large number of suitable comonomers—a desirable property which allows for a large degree of variation in composition, and thereby, materials properties. As well, the styrene hexafluoroalcohol-based polymers are not suitable for imaging with 193 nm radiation due to their opacity at this wavelength, as with other styrenic materials such as PHS. The cyclohexyldodecylfluoroalcohol acrylates also suffer from their high degree of synthetic complexity and high manufacturing expense.
Other photoresists based upon silsesquioxane polymers have also been developed. For example, in commonly assigned U.S. patent application Ser. No. 10/079,289, entitled “Substantially Transparent Aqueous Base Soluble Polymer System for Use in 157 nm Resist Applications,” novel fluorocarbinol- and/or fluoroacid-functionalized silsesquioxane polymers suitable for use in lithographic photoresist compositions are described. Photoresists containing silsesquioxane polymers have also been previously described in U.S. Pat. No. 6,087,064 to Lin et al., U.S. Pat. No. 5,385,804 to Premlatha et al., U.S. Pat. No. 5,338,818 to Brunsvold et al., and U.S. Pat. No. 5,399,462 to Sachdev et al., which disclose the use of aryl or benzyl substituted polysilsesquioxanes in photoresists.
Despite the widespread use of polymeric photoresist materials, as the need for higher resolutions and minimum feature sizes increases, certain characteristics of polymeric resist materials may result in non-uniform pattern features. For example, the non-linear dissolution rates of some polymers, as well as the distribution of polymer chain lengths and chain entanglements, may lead to non-uniform feature dimensions and line edge roughness at very small feature sizes. In turn, such pattern variations may induce fluctuations in threshold voltages and line resistances, thereby degrading circuit performance.
In principle, the use of nonpolymeric materials as photoresists represents a potentially useful approach for avoiding the problems associated with polymeric materials. Since resists derived from single molecules would not contain mixtures of polymer chains of varying lengths, the dissolution characteristics and properties should be uniform. In addition, higher resolutions and decreased line edge roughness may be possible since single molecules have much smaller sizes than polymers and would not suffer from molecular chain entanglement.
Although few examples of nonpolymeric resist materials have been reported to date, the use of certain nonpolymeric materials has been disclosed. In U.S. Pat. No., 6,197,473 to Kihara et al., for example, the use of calixarenes, i.e., cyclic phenolic resins, is disclosed. Photoresists based on calixarenes have also been previously described in, e.g., Ochiai et al. (1997), “High Resolution EB Lithography on Organic resists: Molecular Size Effect,” J. Photopolymer Sci. and Tech. 10(4):641–646; and Fujita et al. (1996), “Nanometer-Scale resolution of Calixarene Negative Resist in Electron Beam Lithography,” J. Vac. Sci. Techol., B14(6):4272–4276.
In U.S. Pat. No. 6,632,582 to Kishimura et al., a siloxane compound is disclosed in which certain substituents may be attached to the silicon atoms, such as an alkyl compound, an ester compound, an ether compound, a sulfone compound, a sulfonyl compound and an aromatic compound. The use of such compounds as resist materials, however, appears to be compromised by the low glass transition temperatures (Tg) of the materials such that the post-exposure bake temperatures would exceed the Tg.
Although improvements in photoresist technology and materials have been made, an ongoing need exists for new photoresist materials and compositions that can provide desirable characteristics for high resolution photoresist applications.